Many applications of nuclear magnetic resonance (NMR) analysis of materials or magnetic resonance imaging (MRI) require a uniform, homogeneous magnetic field over a defined analysis volume. In practice, such analysis volume is usually generated within the interior of a device, and is shaped as a cylinder. Devices that provide such uniform homogeneous magnetic fields over a defined interior volume are also known. One such device, which is illustrated in block diagram form in FIG. 1, is described, for example, in Malcolm McGrieg and Alan G. Clegg, “Permanent Magnets in Theory and Practice”, Second Edition, John Wiley and Sons, 1987, which is incorporated by reference herein as background material.
As shown in FIG. 1, the magnet assembly 10 is constructed of four (or more) pairs of diagonally opposite magnets arranged symmetrically about a substantially cylindrical volume 11 having a longitudinal axis 12, pointing in a direction perpendicular to the drawing sheet. This arrangement of the magnets is designed to create a substantially homogeneous field in the volume 11 defined by the inner periphery of the magnet assembly. In particular, devices of this type have very small spacing (gaps) between adjacent magnets to ensure that the magnetic field is largely uniform over the entire enclosed volume 11. The presence of larger gaps is known to result in field distortions, and thus is generally avoided.
Homogeneous interior field configuration may be provided by using another type of magnetic structure described in German patent DE 3312626 A1, which is hereby incorporated by reference. FIG. 2 illustrates the magnetic structure proposed in this patent, which consists of a cylindrical magnet 100 surrounded by soft magnetic yoke 110. The cylindrical magnet has diametrical direction of magnetization, indicated in the figure by arrow 120. This structure generates a very homogeneous interior magnetic field throughout the inner cylindrical volume 125. The direction of the interior magnetic field is parallel to the direction of magnetization, as indicated by arrow 130.
Permanent magnet assemblies of the type discussed above have a number of advantages, which include no need for field-generating power, low cost of maintenance, and small installation space. However, a problem with devices of this type is that because the magnetic field they generate is largely uniform and homogeneous over the entire enclosed volume, it is unfeasible to analyze only select portions of the enclosed volume and ignore signals from the rest. An industrial application in which such a feature would be desirable is, for example, NMR analysis on a pre-defined interior portion of a cylindrical core, frequently used in material analysis in the process of drilling rocky formations. Therefore, it is perceived that there is a need for a new apparatus and corresponding measurement method that provide such capability.